Spatial scaling, temporal resampling and color format conversion are operations performed at various points of the production and consumption of video. In conventional systems spatial scaling, temporal resampling and color format conversion are each carried out by separate equipment. Resampling operations are common in the industry to provide format conversions including all Digital Television systems such as 1080i, 480i, 576i and other derivatives. Furthermore, while many capture and display systems use the YUV 4:2:2 format, digital compression Codecs (encoders/decoders) associated with such systems commonly work in the YUV 4:2:0 domain, creating a need for high quality conversion.
When scaling digital video, existing spatial scalers resize progressive video as separate frames and interlaced video as separate fields. Each picture is independently scaled in the spatial domain in its own sampling time. For example, 720p pictures are scaled from 1280×720 pictures at 60 Frames/sec; while 1080i pictures are scaled from 1920×540 pictures at 60 fields/sec. Each spatial direction is scaled independently, usually in the horizontal direction first in order to reduce the size of line buffers used for the vertical direction.
There usually are no problems in horizontal scaling, as long as adequately long filters are used. However, for interlaced video there can be quality problems in the vertical domain, even when using sophisticated polyphase filters. The reason for quality problems in the vertical domain is the lack of quality when scaling separate fields due to an implied decimation in the vertical domain that is not accompanied by an antialiasing filter.
Referring to FIG. 1, a diagram 10 is shown illustrating potential aliasing components resulting from vertical spatial scaling of interlaced pictures. Interlaced cameras and video production equipment do not perform antialiasing because any aliasing components are temporary since they are cancelled out when the interlaced material is displayed on an interlaced display device. While vertical spatial scaling of independent pictures works well for progressive video, for interlaced pictures the potential aliasing components (marked by asterisks in FIG. 1) of each field are preserved after resampling and cannot be cancelled out at the display end. Performing antialiasing on the decimated fields yields no new information and reduces perceived resolution that cannot be regained after top/bottom fields are displayed as coherent frames at the display.
Conventional solutions to the problem of high-quality vertical scaling of interlaced content solve the problem by circumventing the problem (i.e., by performing an interlace-to-progressive conversion and operating in the progressive domain). However, performing an interlace-to-progressive conversion and operating in the progressive domain produces twice the amount of data and does not adequately reflect the intended vertical-temporal content of the original video input, especially when the resampled signal is displayed on an interlaced monitor. Furthermore, because interlace-to-progressive conversion is not a perfect operation, artifacts can be created that can be objectionable.